In U.S. Pat. No. 6,179,422 to Lai, an optical tracking device is described to employ two scanning beams to scan across boundaries of a reference mark affixed on an object to be tracked. In an embodiment of eye tracking, the device projects two beams scanning across the limbus at 12 and 3 o'clock positions, respectively. The device detects the timing of the probe beam scanning across the limbus and thus determines the lateral position of the subject's eye.
Two configurations have been described in U.S. Pat. No. 6,179,422 to implement the tracking device. The first one is of open loop, in which the scanning probe beam does not follow the movement of the eye. The second one is of closed loop, in which both the surgical laser beam and the probe beam follow the movement of the tracked eye.
The advantage of the open loop configuration is its simplicity and its feasibility to separate the optical path for position sensing from the optical path for the surgical laser beam. The tracking device can thus be a stand-alone module. Its disadvantage is a limited tracking range due to the curved nature of the limbus, which is the tracking mark for the tracking device of U.S. Pat. No. 6,179,422. The movement detection along two orthogonal directions is no longer independent in an open loop configuration when each probe beam does not intersect perpendicularly with respect to the curved mark such as the limbus.
The advantage of the closed loop configuration is to have much larger tracking range while having both the probe beam and the surgical beam deflected via a common beam steering module. The movement detection along the two orthogonal directions is basically independent in a closed loop configuration because the probe beams have no significant displacement with respect to the limbus. On the other hand, using a common beam steering module for both the surgical and the probe beams introduces a couple of limitations. First, the surgical laser beam has a more complex optical assembly, including the common beam steering module. Second, a bigger mirror is required for the common beam steering module to accommodate both the surgical and the probe beams, while a bigger mirror means a slower system response.